1. Field of the Invention
This invention pertains generally to analog to digital converters and conversion techniques, and more particularly a window tracking apparatus and method for analog to digital conversion.
2. Description of the Background Art
Analog to digital converters are commonly employed to change an analog signal into a collection of digital data bits that accurately represent the signal. While the amplitude of the analog input signal can be continuously variable without discontinuity, the digital output signal into which it is physically changed will contain certain discontinuities which are characteristic of its digital form.
Conventional analog to digital converters measure the amplitude of an analog signal at a given instant during a sampling period and then change the signal to the closest digital form. Therefore, in order to provide for accurate digital representation of an analog signal, the digital form of the signal must change as the analog signal changes. If the sampling rate is too low, the digital form of the signal will not accurately represent the analog signal and low frequency aliasing can occur. It is generally accepted that, in order to convert an analog signal into a usable digital form without aliasing, the sampling rate must be at least twice the highest frequency component of the analog signal. However, the output of a conventional analog to digital converter will contain not only the digital form of the analog signal, but double sideband images of the analog signal centered at multiples of the sampling frequency. Therefore, the output signal must be filtered in order to make it usable. This can require the use of complex filters and filtering techniques in order to eliminate the sideband signals without eliminating a portion of the desired signal.
Furthermore, conventional analog to digital converters are subject to quantization error since, at each sampling instant, there will be a difference between the analog signal and the closest available digital representation. Resolution can be increased by increasing the number of data bits used, but data communications rates between the analog to digital converter and the device receiving the output signal are often limited. As a result, there are practical limitations to the accuracy of conventional analog to digital convertors.
Therefore, there is a need for an analog to digital conversion apparatus and method which overcomes the foregoing limitations of conventional devices. The present invention satisfies that need and overcomes the deficiencies in analog to digital convertors heretofore developed.